Urine samples (24 hr.) were collected for measurement of protein and albumin concentrations before and after hypocaloric diet therapy for a 2-4 month period in seven nondiabetic, seven glucose intolerant and twenty-seven diabetic subjects. After diet therapy, which results in similar percent weight loss in diabetic and nondiabetic subjects, fasting and two hour plasma glucose levels after oral glucose load remained normal in nondiabetics and improved significantly in both the glucose intolerant and diabetic subjects. The concentration of plasma proteins and albumin present in the urine were measured by an immunonephelometric technique using specific antisera against human total plasma protein and albumin. Initially, both mean clearance rates and mean 24 hr excretion of protein and albumin were higher for diabetics than for weight-matched nondiabetic subjects. No changes were found in either total protein or albumin excretion in the nondiabetics. Protein excretion and clearance rates decreased significantly in the diabetic subjects following hypocaloric diet therapy. The findings suggest that hypocaloric diet therapy reduces mild to moderate proteinuria in Type II diabetic patients.